Plants I
Week 4: Plants I
Introduction
Learning Goal: Investigate the colonization of land by plants, examining plant morphology versus algae morphology.
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Learning Objectives:
Recognize and define terms in bold throughout this exercise in your own words.
Apply bolded terms to lab specimens.
Identify specimens based on typical morphological characteristics of their groups.
Illustrate and label key structures of specimens seen in the lab.
Taxonomically categorize lab specimens based on structure.
Apply the basic model of alternation of generations to the reproductive cycles of various phyla of plants.
Historical Context
The colonization of land by plants is considered one of the most revolutionary events in the history of life on Earth, predating the Paleozoic era.
Comparisons indicate that
Extant land plants are related to green algae, particularly a group called charophyceans.
Both green algae and land plants share characteristics:
Presence of chlorophyll a and b
Cell wall composition: made of cellulose
Storage of excess carbohydrates as starch produced in amyloplasts.
Molecular and ultrastructural data suggest land plants most likely descended from charophyceans, forming a group called Streptophyta.
Synapomorphies distinguishing Streptophytes include:
Plasmodesmata
Anisogamous sexual reproduction
A unique form of cytokinesis.
Distinguishing Features of Land Plants
Land plants are a subgroup of Streptophytes, with distinguishing features such as:
Meristems: Continuously growing tissues and cell division locations.
Alternation of generations: A reproductive cycle that alternates between generations.
Maternally dependent embryos: Ensures protection during development.
Sporangia and gametangia: Specialized structures for producing reproductive cells.
Desiccation-resistant structures: Protect reproductive cells during dispersal.
Environmental Challenges and Adaptations
Primitive land plants faced severe environmental challenges compared to algae, including:
Variations in temperature, moisture, substrate, and gravitational forces.
New adaptations emerged, including:
Rhizoids and later roots: Adapted to absorb nutrients and minerals from soil.
Development of gametangia: Protects embryos from desiccation.
Presence of a waxy cuticle: Prevents water loss, though also hinders gas exchange—a problem solved with stomata.
Development of vascular tissue: Efficiently transports materials throughout plant bodies.
Reproductive Cycle
Alternation of Generations
Most land plants show a common sexual reproductive cycle called alternation of generations, including:
Alternation between haploid gametophyte generation and diploid sporophyte generation.
Reproductive Processes:
Gametes: Include sperm and ova.
Sporophyte generation: Dominant generation in most present-day land plants (except non-vascular like liverworts, hornworts, and mosses).
Meiosis: Sporophytes produce haploid spores in sporangium.
Germination: Spores lead to haploid gametophytes, which produce haploid gametes via mitosis.
Fertilization: Process of egg and sperm fusion to form a diploid zygote, which develops into a multicellular sporophyte.
Emphasis on differences in the life cycle:
Spores: Formed by meiosis; lead to gametophyte plants.
Gametes: Formed by mitosis; fuse to form zygotes.
Reproductive Assurance
Some plants can reproduce sexually and asexually; frequency depends on population genetics.
Asexual reproduction: - Can reduce genetic diversity, potentially making populations less adaptable.
However, it provides reproductive assurance in low-gamete union conditions.
Sexual reproduction: Generally promotes evolutionary adaptation, though it’s slower and costlier than asexual methods.
Plant Phyla Overview
According to Campbell Biology, there are ten extant phyla of plants, categorized in four main groups:
Nonvascular Plants:
Includes liverworts (Phylum Marchantiophyta), mosses (Phylum Bryophyta), and hornworts (Phylum Anthocerotophyta).
These lack complex vascular tissues and grow mainly in moist habitats.
Seedless Vascular Plants:
Include lycophytes (Phylum Lycopodiophyta) and pteridophytes (Phylum Pteridophyta), adapted for life away from water using vascular tissues.
Gymnosperms:
Seed-producing plants without flowers or fruits, characterized by seeds exposed on cones.
Angiosperms:
Flowering plants that produce seeds enclosed within a fruit.
Major Themes Recap
Adaptations for Terrestrial Life:
Cuticle, stomata, lignin, vascular tissue, roots, pollen.
Structure-Function Relationships:
Gas exchange regulation by cuticle and stomata; nutrient absorption by roots; transport via vascular tissues; structural support by lignin.
Evolutionary Trends:
Transition from aquatic to terrestrial life; development of vascular tissues; evolution of roots, leaves, and stems; progression from spores to seeds; and mechanisms for pollination.
Reproductive Cycle Modifications:
Bryophytes: Small gametophytes with sporophyte dependency.
Ferns: Larger sporophytes with reduced gametophytes.
Gymnosperms: Dominant sporophyte generation.
Seedless Non-Vascular Plants
Learning Goal
Expand understanding of non-vascular plant form and function to discuss the role of structures in land colonization.
Learning Objectives:
Recognize, define, apply, identify, illustrate, categorize non-vascular plants and their structures.
Contrast sexual and asexual reproduction.
Apply alternation of generations to reproductive cycles of non-vascular plants.
Predict evolutionary steps needed for land colonization based on observed structures.
Nonvascular Plants Classification
**Phyla of Nonvascular Plants: **
Marchantiophyta: Liverworts
Bryophyta: Mosses
Anthocerotophyta: Hornworts
The life cycle is gametophyte dominant, remaining close to the ground facilitating sperm mobility for fertilization.
Lack of true vascular tissue means limited distribution to moist environments.
Features:
Absence of true roots, stems, and leaves, with rhizoids providing anchorage and some absorptive functions.
Life Cycle of Nonvascular Plants
Fertilization Process:
Male gametangium (antheridium) produces sperm that swim towards the egg in the archegonium when water is present.
The zygote (2n) grows into an embryo (2n) nurtured by the female gametophyte (n).
The sporophyte forms from the archegonium after fertilization and remains attached.
Meiosis: Occurs in the sporangium to produce spores, which are released into the environment when mature.
Spores germinate to develop young gametophytes.
Bryophytes may have separate male and female gametophytes biologically distinct.
Phylum Marchantiophyta - The Liverworts
Commonly known as liverworts, they reproduce both sexually and asexually:
Asexual reproduction: Via fragmentation or gemmae cups.
Gemmae Cups: Located on the thallus, releasing gemmae that can grow into new gametophytes.
Structure Observations
Review and label structure:
Thallus: The gametophyte generation.
Rhizoids: Anchor the plant; some nutrient absorption.
Reproductive Structures
Antheridia: Male sex organs producing sperm, resembling umbrellas.
Archegonia: Female sex organs resembling tiny banana trees.
Examine, sketch, and label antheridia and archegonia, focusing on their morphology.
Sporophyte Development: Forms from fertilization occurring at archegonia, remaining attached to female gametophyte.
Seedless Vascular Plants
Learning Goal
Deepen understanding of the role of vascular seedless plants in diverse terrestrial environments.
Learning Objectives:
Recognize and apply key terms to specimens,
Identify morphological characteristics of seedless vascular plants,
Illustrate and label key structures,
Distinguish vascular tissue forms and functions,
Apply alternation of generations to their reproductive cycles.
Introduction to Vascular Plants
All vascular plants contain vascular tissue enabling true leaves, stems, and roots.
Functionality of vascular tissue:
Xylem: Transports water and minerals from roots to aerial parts.
Phloem: Transports sugars from leaves to other plant parts.
Vascular Tissues and Water Transport
Stomata: Openings in leaves allowing CO2 absorption for photosynthesis, though leading to water loss (transpiration).
Large trees can manage up to 100 gallons of water daily through transpiration processes.
Guard Cells: Mechanism for stoma opening/closing based on water availability.
Phylum Monilophyta – Ferns
Ferns represent a significant group within vascular seedless plants, thriving in varied habitats.
Distinct features:
Independent sporophyte phase with vascular tissues and stomata present.
Observational Task:
Sketch and label components: Frond, Fiddlehead, Rhizome, Sori (clusters of sporangia on leaf undersides).
Homosporous: Most ferns produce a single type of spore that develops into one gametophyte producing both archegonia and antheridia.
Specialized Structures of Ferns
Frond: Main leaf structure of the fern.
Sori: Clusters on the undersides of fronds containing sporangia.
Fertilization requires water for sperm to swim towards the egg in archegonia.
Phylum Lycophyta (Lycopodiophyta)
General Overview
Lycophytes, known as club/spike mosses but distinguishable by vascular tissue.
Reproduction:
Homosporous club mosses versus heterosporous spike mosses.
Examination Task:
Draw examples: Lycopodium sp. (club moss), Selaginella sp. (spike moss).
Identification of leaf morphology:
Noted for needle-like or scale-like leaves with a single unbranched vein.
Conclusion
The study of plant morphology, reproduction, and evolution provides insight into the remarkable adaptations that allowed plants to thrive on land. By understanding the structures, reproductive cycles, and taxonomic classifications, students can appreciate the complexity and diversity of plant life that has emerged over millions of years of evolution.